1. Field of the Invention
The present invention relates to a semiconductor device having a function for controlling current supplied to a load with a transistor, and relates to a display device including a pixel which is formed of a display element, luminance of which is changed in accordance with a signal, and a signal line driver circuit or scan line driver circuit which drives the pixel. In addition, the present invention relates to a driving method of such a semiconductor device and a display device. Further, the present invention relates to an electronic device having the display device in a display portion.
2. Description of the Related Art
In recent years, a self-luminous display device having a pixel formed by using a light-emitting element such as an electroluminescence (EL) element, i.e., a so-called to as light-emitting device has attracted attention. As a light-emitting element which is used for such a self-luminous display device, an organic light-emitting diode (OLED) and an EL element have attracted attention, and they have been used for an EL display or the like. Since these light-emitting elements emit light by themselves, an EL display or the like has advantages compared to a liquid crystal display such that it has higher pixel visibility, no backlight is needed, and response speed is higher. Note that luminance of a light-emitting element is often controlled by a current value flowing to the light-emitting element.
In addition, an active matrix display device in which a transistor which controls light emission of a light-emitting element is provided in each pixel has been developed. The active matrix display device has been expected to be put into practical use because not only it can realize high-definition display and large-screen display which are difficult to realize in a passive matrix display device, but also it can operate with less power consumption than the passive matrix display device.
FIG. 62 shows a pixel structure of a conventional active matrix display device (see Reference 1: Japanese Published Patent Application No. H08-234683). The pixel shown in FIG. 62 includes a thin film transistor (TFT) 11, a TFT 12, a capacitor 13, and a light-emitting element 14, and is connected to a signal line 15 and a scan line 16. Note that a power supply potential Vdd is supplied to one of a source electrode and a drain electrode of the TFT 12 and one electrode of the capacitor 13, and a ground potential is supplied to an opposite electrode of the light-emitting element 14.
At this time, in the case of using amorphous silicon for a semiconductor layer of the TFT 12 which controls a current value supplied to the light-emitting element 14, i.e., a driving TFT, the threshold voltage (Vth) fluctuates due to deterioration or the like. In that case, although the same potential is applied from the signal line 15 to different pixels, current flowing to the light-emitting element 14 is different in each pixel, and display luminance becomes ununiform depending on the pixels. Note that also in the case of using polysilicon for the semiconductor layer of the driving TFT, characteristics of the transistor deteriorate or vary.
In order to overcome this problem, an operating method using a pixel in FIG. 63 is proposed in Reference 2 (Reference 2: Japanese Published Patent Application No. 2004-295131). The pixel shown in FIG. 63 includes a transistor 21, a driving transistor 22 which controls a current value supplied to a light-emitting element 24, a capacitor 23, and the light-emitting element 24, and is connected to a signal line 25 and a scan line 26. Note that the driving transistor 22 is an NMOS transistor, and a ground potential is supplied to one of a source electrode and a drain electrode of the driving transistor 22 and Vca is supplied to an opposite electrode of the light-emitting element 24.
FIG. 64 shows a timing chart of operations of this pixel. In FIG. 64, one frame period is divided into an initialization period 31, a threshold voltage (Vth) writing period 32, a data writing period 33, and a light-emitting period 34. Note that one frame period corresponds to a period for displaying an image for one screen, and the initialization period, the threshold voltage (Vth) writing period, and the data writing period are collectively referred to as an address period.
First, the threshold voltage of the driving transistor 22 is written into the capacitor 23 in the threshold voltage writing period 32. After that, data voltage (Vdata) showing luminance of the pixel is written into the capacitor 23 and Vdata+Vth is stored in the capacitor 23 in the data writing period 33. Then, the driving transistor 22 is turned on in the light-emitting period 34, so that the light-emitting element 24 emits light with luminance specified by the data voltage by changing Vca. By performing such an operation, variations in luminance caused by fluctuations of the threshold voltage of the driving transistor 22 are reduced.
Reference 3 (Reference 3: Japanese Published Patent Application No. 2004-280059) also discloses that voltage of the sum of the threshold voltage of a driving TFT and a data potential corresponds to gate-source voltage of the driving TFT, so that current flowing to a light-emitting element does not change even when the threshold voltage of the TFT fluctuates.
As described above, in a display device, suppression of variations of a current value caused by variations in the threshold voltage of a driving TFT has been expected.
In each of the operating methods disclosed in Reference 2 and Reference 3, initialization, threshold voltage writing, and light emission are performed by changing a potential of Vca several times in each one frame period. In each pixel disclosed in Reference 2 and Reference 3, since one electrode of a light-emitting element to which Vca is supplied, i.e., an opposite electrode thereof is formed over the entire pixel region, the light-emitting element cannot emit light if there is even one pixel in which a data writing operation is performed other than initialization and threshold voltage writing. Therefore, as shown in FIG. 65, a ratio of a light-emitting period in one frame period (i.e., a duty ratio) becomes low.
When the duty ratio is low, the amount of current supplied to a light-emitting element and a driving transistor is needed to be increased, so that voltage applied to the light-emitting element becomes higher and power consumption also becomes higher. In addition, since the light-emitting element and the driving TFT easily deteriorate, screen burn-in is generated or higher power is needed to obtain luminance which is almost equal to luminance before deterioration.
In addition, since the opposite electrode is connected to all pixels, the light-emitting element functions as an element having large capacitance. Therefore, in order to change a potential of the opposite electrode, high power consumption is needed.